Anoxia of the heart causes failure of contraction prior to any irreversible injury. The mechanism by which anoxia blocks cardiac excitation-contraction coupling is unknown. Studies in whole muscle are confounded by heterogeneity; however-it has not previously been possible to achieve the low oxygen tensions required to study anoxia in single myocytes during electrophysiology recordings. Guided by calculations of oxygen transport, we developed a system in which myocytes in an open dish are insulated from oxygen by a laminar counterflowing column of argon, permitting free access by microelectrodes while maintaining a PO, less than 0.02 torr. In the absence of glucose, the-amplitude of stimulated contraction of anoxic ventricular myocytes fell. to zero over two minutes, after a lag period attributable to consumption of endogenous glycogen. The cytosolic calcium transient, measured by Indo-1 fluorescence, fell to zero simultaneously with the contraction. After the twitch had failed, microinjection of caffeine around the cell still caused a large calcium release and contraction, indicating that sarcoplasmic reticulum calcium stores were not depleted. Failure of the twitch was accompanied by shortening and then failure of the action potential; under voltage clamp, large outward currents, reversing at the resting potentia , developed during contractile failure. After failure of action potential -mediated contraction, voltage clamp depolarization, using a large command voltage to compensate for the series resistance error due to the outward currents, restored a normal twitch contraction. We conclude that anoxic contractile failure in the rat myocyte is due to alteration of the action potential, and the distal pathways of excitation-contraction coupling remain essentially intact.